Add spec for `defined?` on constant with const_missing #1803

Job	Run time
RuboCop	31s
specs (ubuntu, 3.0.6)	1m 27s
specs (ubuntu, 3.1.4)	1m 32s
specs (ubuntu, 3.2.2)	1m 24s
specs (ubuntu, 3.3.0)	1m 25s
specs (macos, 3.0.6)	1m 49s
specs (macos, 3.1.4)	1m 21s
specs (macos, 3.2.2)	1m 14s
specs (macos, 3.3.0)	1m 14s
specs (windows, 3.0.6)	4m 19s
specs (windows, 3.1.4)	4m 57s
specs (windows, 3.2.2)	5m 13s
specs (windows, 3.3.0)	4m 31s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 24s
	32m 21s

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